1. Field of the Invention
The present invention relates to optical shutters for controlling the transmittance of light used in holographic memory units or optical communications devices and, in particular, to an optical shutter capable of varying the transmittance of light by means of a slight operation.
2. Description of the Related Art
Presently, mechanical optical shutters that control transmission/blocking of light are known and widely used in optical devices such as optical memory units, optical logic units, and optical communications devices. Such a conventional mechanical optical shutter is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 9-218360. The mechanical optical shutter controls transmission/blocking of light by operating light-shielding plates disposed in an optical path to open or close the optical path. Aside from such an optical shutter that opens or closes the optical path by parallel movement of the light-shielding plates, there are other mechanical optical shutters that open or close optical paths by rotating light-shielding plates. In either case, light-shielding plates are operated in order to open or close the optical path.
In memory units using holographic principles (hereinafter referred to as “holographic memory units”), pulsed light is used as a reference beam and a beam to illuminate an object and the pulsed light is formed by using optical shutters. In this case, higher response speed is preferable.
Conventional mechanical optical shutters, however, require a minimum movement on the order of 10 μm for the light-shielding plates in order to open or close the optical paths. Each of the light-shielding plates, even a minute one, has a certain weight, and therefore, the response speed is limited to about 200 μs. Since holographic memory units need a response speed of 50 μs or less, it is necessary to significantly enhance the response speed over that achievable by conventional techniques. Optical shutters achieving high response speed are desired in, for example, the field of optical communications, as well as in holographic memory units.
In conventional mechanical optical shutters, the mechanisms required for driving the light-shielding plates are inevitably large due to the large distances that the light-shielding plates must travel. For light-shielding plates driven by piezoelectric elements, increased thicknesses of these piezoelectric elements are necessary to move the light-shielding plates by a large distance because the light-shielding plates expand or shrink in a predetermined ratio in accordance with their thicknesses. Consequently, the overall units are unavoidably large.